I would have to say that the most importnant factor is learning a new skill or improving an existing one, because a person needs to constantly keep learning in order to be ahead of others. So, that's my number 1. Number 2, in my opinion, is - having the change to mix and make new friends, as it helps to improve the person's communication skills (which might be useful in the future). Then, as number 3, comes having fun and avoiding boredom, since it's a summer activity course. And the last but not the least - improving fitness and/or strength. I put it on the 4th place, as it's not the key point of spending your time efficiently in summer.
In my honest opinion, the most important one is to upgrade your learning skills. 'Cause summer vacation lasts about 3 months and you can forget all the learned material very easily and fast. Even when your lessons in Math or English take you about 30-40 min a day, it will help you to memorize everythimg you've already known and probably improve your knowledge. Of course, having new friends during the summer is very important for all of us, too. In summer, we, first of all, have a break after studying. You can learn more new and interesting people, getting good relation- or friendship with them. Isn't it a great experience? However, it would be cool, if you don't just upgrading your learning skills or relationships with others, but yourself too. Doing exercises and having fitness monitoring will help you for sure to stay slim and fit.
